The present invention relates to pneumatic tires and particularly tubeless pneumatic tires comprising a carcass of cord reinforced rubber fabric. The invention is particularly but not exclusively applicable to radial tires in which the carcass cords extend radially between two bead regions.
Conventional pneumatic tires have a carcass comprising one or more plies of tire fabric consisting of a plurality of parallel reinforcing cords embedded into a thin film or sheet of rubber.
In modern radial tires the carcass comprises one or more plies having cords in a radial configuration, whereas in older cross-ply constructions two or more plies of tire fabric have their cords disposed at an angle to the radial direction and in crossing relationship to the cords of adjacent plies.
Materials suitable for cords of tires are varied and include organic materials such as rayon, nylon, polyester or aramid, inorganic materials such as glass or metals such as steel. Whichever material is used for the cord the tire manufacturer endeavors to maximize the structural integrity of the tire throughout its life by embedding the cords in suitable rubber compound known as a topping rubber which is particularly formulated to give good adhesion to material of the cord.
The carcass of a tubeless pneumatic tire is characterized by a rubber innerliner which is provided on the inside of the tire surface radially inwardly of the carcass ply for the purpose of retaining the inflation medium when the tire is mounted on a wheelrim. Accordingly the innerliner conventionally comprises a compound of a rubber which has low permeability to air, such as butyl rubbers or halo-butyl rubbers including bromo-butyl and chlorobutyl rubbers. The formulation of innerliner rubbers is directed towards maximizing impermeability to air and the resultant compounds have little or no adhesion to the materials of tire cords.
The manufacture of a tire commonly comprises a three-stage process. In the first stage the uncured rubber components of the carcass are laid sequentially around the outer surface of a drum to form a right-cylindrical assembly. This assembly includes a fabric of reinforcing cords which are laid over a thin sheet of rubber liner and anchored in each of the two edge regions of the cylinder which will ultimately form the bead regions of the tires. In radial tire manufacture the carcass ply cords extend perpendicularly between the edges of the right-cylindrical assembly.
In the second stage of manufacture the first stage right-cylindrical assembly is deformed into a toroidal shape by radially outwardly expanding the central portion of the cylinder whilst simultaneously moving the edge portions nearer to each other. The remaining components of the tread region are then assembled onto the crown of the toroid to complete assembly of the uncured tire.
In the final stage of manufacture the uncured tire is placed within a heated profile mould and given further slight radial expansion to form the tread pattern and finally vulcanize the tire assembly transforming it from a plastic to an elastomeric state.
A common problem which can arise in tire manufacture concerns the length of the cord path between the edges of the right-cylindrical carcass assembly in the first stage of manufacture. If this length is too short, or if the cord path length or cord extensibility varies significantly around the periphery of the cylinder then during the above-mentioned subsequent expansion of the carcass the cords may tighten excessively such that the rubber material radially inward of the cords is pushed outward through the spaces between adjacent cords. Thus effectively the carcass cords are "pulled through" their cord topping rubber into the rubber below. In a radial tire carcass this cord "pull through" is apt to occur locally in the tire shoulder region beneath and outward of the breaker edge where the carcass curvature increases most rapidly.
In extreme cases the carcass cords may pull all the way through to the interior surface of the tire at which occurrence the tire is scrapped on routine visual examination. However potentially more serious are cases where the carcass cords pull through to a lesser extent only into the rubber liner. Such occurrences may well not be evident on visual inspection of the tire but the lack of adhesion between the carcass cords and the rubber poses a severe risk of subsequent premature failure of the tire carcass in service.